© Daniel J. Cox/Natural Exposures
1/15/2013 6:50:12 AM
Polar Bears on the Western Coast of Hudson Bay in November, 2011 & 2012
By Ian Stirling, Daryll Hedman, and Vicki Trim
The ecological background
In late fall, polar bears in the Western Hudson Bay subpopulation accumulate along the shoreline waiting for freeze-up so they can return to the sea ice, which is the critical platform they need for hunting seals. Following breakup of the sea ice several months earlier, there is no ice on Hudson Bay so the bears have not had much to eat. Instead, they have to survive on whatever fat reserves they accumulated before the sea ice melted.
However, for the polar bears of western Hudson Bay, the average date of breakup is now a full 3 weeks earlier than it was only 30 years ago! The reason this change is so critical is that late spring and early summer is the period when the greatest numbers of fat, newly weaned ringed seal pups are available, and are probably easier to catch because of being less experienced with polar bears. The bears take on an estimated two-thirds or so of the energy they will need for the entire year during the spring feeding period prior to breakup. Thus, it is a double whammy for the bears – not only are they unable to access seals in the open water, but they also lose the ice platform they need for hunting at the most important time of the year for feeding. Then, they have to survive for longer and longer periods with less stored fat because freeze-up in the fall is becoming later as well.
Because of the factors described above, we now understand why body condition of bears of all age and sex classes has declined significantly over time in Western Hudson Bay. The average weight of lone, probably pregnant, adult female polar bears in the fall has declined by about 40 or more kilograms between 1980 and 2007. As a result, females are having fewer cubs and their average weights over time have declined. The cubs of lighter females have higher mortality rates than those born from heavier females.
In November 2011 and 2012, we counted polar bears along the coast of western Hudson Bay between the Manitoba/Ontario Border in the south to La Perouse Bay to the west of Cape Churchill. We wanted to document their distribution prior to returning to the sea ice, determine if there were any large aggregating groups, classify them by age and sex class, and estimate their body condition. We did the surveys just as the sea ice was beginning to form along the coast in order to maximize the number of bears available to be counted.
Because the surveys were conducted only along the coast, they could not be used to estimate the total size of the population. However, the proportion of bears of different age and sex classes can be used as an index of population structure, in that area, at that time of year. In particular, the proportion of cubs-of-the-year and yearlings surviving the open water period and still with their mothers just before freeze-up provide valuable insight into the potential for subsequent recruitment into the population.
It is easy from the air to identify large adult males, and females accompanied by dependent offspring (cubs-of-the year or yearlings). Similarly, subadults (mainly 2 and 3-year-olds but likely including a few 4-year-olds) are easy to recognize from their size and the way they run, although their sex cannot be determined. However, separation of adult females and young adult males is not sufficiently reliable to justify separate tabulation so for the purposes of these surveys, we called them “U-bears”, or unclassified.
Lastly, we also wanted to assess the possible value of such surveys to give an approximate index of the overall condition of bears of different age and sex classes, just before freeze-up when the bears could return to the sea ice to hunt seals. We estimated the condition of the bears visually on a subjective scale of 1-5, where 1 is the thinnest and 5 is the fattest. Although the thinnest and fattest bears are easy to identify, the greatest degree of overlap is probably between bears classified as “3”, or pretty much average condition, and those identified as “2”, which are thin but not yet a bag of bones that would be called a “1”. This is because in some cases, the heavy coat of the polar bear makes it difficult to see how thin a bear actually is. Consequently, our estimates of thin bears are likely to be an underestimate but the degree of actual error is difficult to define.
What did we see?
In both years, we were unable to survey the whole area we wanted to because of poor weather conditions in part of the survey area. In 2011, much of the coastline between La Perouse Bay and approximately 60 km to the south was obscured by wind and blowing snow while in 2012, we could not proceed south from about the Kaskatama River to the Manitoba/Ontario border because freezing rain made it unsafe to fly. In both years, sections of the coast already had variable amounts of unconsolidated ice extending up to a few km offshore so that an unknown number of bears had likely already left the shoreline.
In 2011 we counted 156 bears, of which 4 (2.6%) were cubs-of-the-year and 5 (3.2%) were yearlings. We collected specimens from one cub that appeared to have starved and was being fed upon by two U-bears. One female with two cubs was so thin she could barely stand up when approached. In 2012, we counted 123 bears, of which 11 (8.9%) were cubs-of-the-year but there were no yearlings at all (i.e., 0%).
In 2011 and 2012, we counted 31 (19.9%) and 30 (24.4%) U-bears respectively, of which a large (but unknown) proportion were almost certainly adult females that did not have cubs or yearlings that year at all or had lost them prior to freeze-up. In both years, we saw only a very few bears at sites where, 20 or more bears regularly congregated each fall only 20 years ago.
What did we learn?
The very low proportion of yearlings in our 2011 count was consistent with observations made during a much larger study that surveyed both coastal and inland areas in Manitoba in early September by the Government of Nunavut. They counted a total of 711 bears, of which only 7% and 3% respectively were cubs-of-the-year and yearlings. To emphasize the significance of these observations, and their concern about the low numbers of cubs, and especially yearlings, still with their mothers, the authors of the Nunavut report summarized comparative data from recent large-scale population assessments in other areas where the bears are also forced to fast for several months on their stored fat reserves: Foxe Basin, Southern Hudson Bay, Davis Strait, and Baffin Bay. However, the proportion of cubs seen in those studies ranged from 8-19% and the proportion of yearlings from 9-12% (Table 5). Clearly, the proportion of cubs of the year in western Hudson Bay was lower overall than in other areas but the proportion surviving through their first year of life to become yearlings, in 2011 and 2012 at least, is dismal.
Although other aspects of the results from the survey are still being evaluated, two important aspects seem clear.
1) If the proportion of cubs surviving through their first year of life continues to be as low as in 2011 and 2012, it is unlikely there will be adequate recruitment to sustain the polar bear population of western Hudson Bay under present environmental conditions, even without removals of problem bears or harvested animals.
2) Our relatively small-scale surveys appeared capable of providing sufficiently large sample sizes to provide results on the proportions of cubs and yearlings in the population that were consistent with those from a much larger independent aerial survey.
The surveys were conducted by Daryll Hedman and Vicki Trim from the Manitoba Department of Natural Resources, and Ian Stirling, a scientific advisor to Polar Bears International. Financial and logistic support for the surveys was provided by Polar Bears International and the Manitoba Department of Conservation and Water Stewardship. Justin Seniuk from Prairie Helicopters provided safe flying throughout the surveys.